This invention is in the field of Planar Lightwave Circuits (PLC), and relates to a multilayer integrated optical device and a method of fabrication thereof.
Optical communications is the enabling technology for the information age, and the essential backbone for long haul communications. As this technology progresses, there is a tremendous interest in providing optical routes in the short haul, metropolitan and access networks, as well as in local area networks and cable TV networks. In all these networks, the best of breed solution for bandwidth expansion has been the adoption of wavelength division multiplexing (WDM), which entails the aggregation of many different information carrying light streams on the same optical fiber. Devices capable of accessing individual information steams are fundamentally required in current and future networks. These devices can also add information streams to the optical fiber, as well as impress information on an optical stream by optical modulation.
PLC technology is central in the creation of modern optical elements for communications systems. According to this technology, optical waveguides and additional functional structures are fabricated in a planar optical transparent medium in order to direct the passage of light and to implement coupling, filtering, switching, and additional processing functions as required for optical communications.
Existing examples of Planar Lightwave Circuits include optical switches and modulators based on Mach Zender Interferometer (MZI), in which interference is produced between phase coherent light waves that have traveled over different path lengths arrayed waveguide routers (AWG) used for combining and spreading multiple optical channels, namely multiplexers and demultiplexers. However, to achieve a good modulation performance with the MZI, the latter is typically designed with long interference arms. As a result, this device is not size-efficient in its implementation, and limits the scaling ability of complex optical circuits. Another feature of MZI-type devices, in their predominant implementation, is their frequency insensitivity over a desired frequency bandwidth. As a result; MZI-type devices cannot be used directly for wavelength routing.
An important driving force pushing PLC technology is the need for enhanced functionality in the optical domain. This need is hampered by the limitation of state of the art waveguide technology, which is two-dimensional (i.e., single-functional-layer architecture). Unlike the very large scale integrated electronic circuitry, where dimensions of the basic elements were reduced to sub-micrometer size, the optical PLC circuitry is inherently much larger, thus the exploitation of multi-layer architectures is much more crucial than in electronics.
In the implementation of PLC, there is a contradiction between the requirements of coupling to optical fibers and decreasing circuit size. Coupling to fibers is best obtained by using waveguides with modal fields similar to the fiber modes with a small refractive index difference with respect to the surrounding medium. The functionality of the optical circuits depends on the amount of optical elements in the circuit. By decreasing the circuit size, more optical circuits can be integrated and the attainable functionality increases. Smaller dimensions imply tighter control of the optical mode and smaller optical modes, hence, a high index contrast between the waveguide core and surrounding medium. It is of fundamental importance to provide a means of combining both elements in one functional optical circuit.
The importance of utilizing the vertical dimensions in creating complex optical circuits has been recognized and addressed in the past. This is associated with the fact that vertical fabrication tolerances are better than horizontal tolerances, and therefore such a vertical integrated optical device, (filter, switch, modulator) is simpler or cheaper to manufacture. Optical devices utilizing this approach are disclosed, for example, in the article xe2x80x9cVertically Coupled Glass Microring Resonator Channel Dropping Filtersxe2x80x9d, B. E. Little et al., IEEE Photonics technology Letters, Vol. 11, No. 2, February 1999. This approach is critical for the fabrication of optical circuits based on structures with very different indices of refraction such that the effective coupling region between the structures is very small, e.g., coupling between waveguides and ring micro-resonators. In this case, the vertical dimension, which is easier to control in conventional processes, can mediate the structure for accurate coupling as described in the aforementioned reference.
Recently developed integrated electo-optical devices utilize resonant rings to achieve frequency selective switching. Such a device is disclosed, for example, in WO 99/17151. The device comprises a ring resonator interconnected by linear waveguides to couple light from a first linear waveguide to the second one, when the frequency of the light passing through the first waveguide fulfils that of the resonance condition of the ring. By applying an electric field to the ring, its refractive index, and consequently, its resonance condition can be desirably adjusted, thereby preventing the passage of the previously coupled light, the device therefore acting as a switch. Alternatively, the loss of the ring waveguide can be changed. Adding loss to the ring diminishes its operation as a resonant cavity, and light cannot be coupled from the waveguide to waveguide.
To create two or more layers of interconnected waveguides with the prior art techniques, a planarization step has to be performed. FIGS. 1A-1D illustrate main sequential steps of the prior art technique employed for manufacturing a waveguide structure shown in FIG. 1E being generally designated 10. Initially, a buffer layer 12 of SiO2 is deposited on a silicon wafer 14 (FIG. 1A). Then, a layer 16 of doped SiO2 with a higher refractive index (SiO2+Ge), as compared to that of the buffer layer 12, is deposited onto the buffer layer (FIG. 1B). This layer 16 serves for the formation of a core 16A of the optical waveguide, and its thickness is typically in the range of 4-12 xcexcm. To form the waveguide core 16A (FIG. 1C), the waveguide, as well as other optical structures, are masked using photolithography followed by etching. A third layer of SiO2, or upper cladding layer 18, is then deposited so as to bury the etched structure (FIG. 1D).
This layer 18 retains to some extent the topography of the underlying structure, and thus requires planarization to allow for an additional overlaying waveguide structure to be deposited. Planarization can be implemented by Chemical Mechanical Polishing, reflow techniques, deposition of a very thick layer, selective etching or deposition techniques. As shown in FIG. 1F, after achieving a planar top layer, a second waveguide structure 20 can be fabricated on top of the structure 10 in the above-described manner.
Planarization is a difficult process step, which utilizes expensive equipment and is difficult to be accurately applied for large area wafers. Therefore, it would be desirable to eliminate this step in the fabrication of multi-layered optical waveguide structures.
There is accordingly a need in the art to facilitate the manufacturing of a three-dimensional (multi-layered) integrated optical device, by providing a novel method of fabricating such a device, and a novel integrated optical device based on an optical structure embodying different material systems. Such a device may be an optical frequency dependent switch, a modulator, an Optical Add Drop Multiplexer (OADM), a spectral analyzer, a sensor, etc.
The main idea of the present invention consists of utilizing a waveguide definition on several layers, enabling to combine low coupling loss waveguides with high confinement waveguides. The present invention opens new horizons for the functionality of optical devices using Planar Waveguide Technology. The present invention provides a fabrication method for the manufacture of three-dimensional fabrics of waveguides with three-dimensional interconnections. Furthermore, since it is recognized that three-dimensional interconnections are crucial for creating resonator-based, high-density optical circuits, the invention provides a fabrication method for such devices.
The invention provides for the fabrication of three-dimensional optical waveguiding structures by simple processing steps. The main innovation here relates to the elimination or at least alleviation of the planarization step, which is difficult to implement. As indicated above, planarization is required to overcome the perturbations in a given layer caused by the previously deposited layers. In the present invention, the adoption of novel waveguide structures minimizes the perturbation, and facilitates multi-level integration of light guiding structures.
There is thus provided according to one aspect of the present invention, a method of fabricating an integrated optical device comprising a structure including at least one waveguiding element, the method comprising the steps of:
(i) forming a basic structure containing a substrate material carrying a buffer material layer coated with a core material layer of a higher refraction index as compared to that of the buffer layer;
(ii) defining said at least one waveguiding element in a guiding layer on top of said basic structure, wherein said guiding layer is made of a material with a refractive index higher than the refractive index of said buffer layer and the core layer, and is chosen so as to minimize a height of said at least one waveguiding element and to provide effective guiding of light in the core layer;
(iii) forming a cladding layer on top of a structure obtained in step (ii), wherein a height difference between a height of the cladding layer region above said at least one waveguiding element and a height of the cladding layer region outside the waveguiding element is substantially small, thereby providing a sufficient flatness of the top cladding layer to allow formation of a further waveguide structure thereon and prevent significant perturbation in light propagation within said further waveguide structure.
At least one waveguide element may be defined by a ridge of the high index material (as compared to the buffer layer) on top of the basic structure. Alternatively, this waveguide element may be a resonator ring (resonator cavity loop), in which case the further waveguide structure contains a further waveguiding element formed on top of the cladding layer by repeating steps (i) and (ii).
It should be understood that the term xe2x80x9csufficient flatness of the cladding layerxe2x80x9d used herein signifies a flatness defined by the height difference of the different regions of the cladding layer (i.e., above the waveguiding element and outside the waveguiding element) much smaller than the optical mode zise of a further waveguiding element.
For example, the height difference in the order of few hundreds of nanometers can be obtained, the optical mode size of the further waveguide formed on the cladding layer being about several micrometers. Typically, the height difference of the cladding layer does not exceed the height of the at least one waveguiding element (i.e., the thickness of the guiding layer) covered by said cladding layer.
According to another aspect of the present invention, there is provided a method of fabricating a three-dimensional integrated optical device comprising at least two vertically aligned waveguide structures, each including at least one waveguiding element, the method comprising the steps of:
(a) forming a basic structure containing a substrate carrying a buffer layer coated with a core material layer of a higher refractive index as compared to that of the buffer layer;
(b) defining said at least one waveguiding element of the lower waveguide structure in a guiding layer on top of said basic structure, wherein said guiding layer is made of a material with a refractive index higher than the refractive index of said buffer layer and the core layer, is chosen so as to minimize a height of said at least one waveguide element and to provide effective guiding of light in the core layer; and
(c) forming an upper cladding layer on top of a structure obtained in step (b), wherein a height difference between a height of the cladding layer region above said at least one waveguiding element and a height of the cladding layer region outside the waveguiding element is substantially small, thereby providing a sufficient flatness of said upper cladding layer to allow direct formation of the upper waveguide structure thereon;
(d) forming said upper waveguide structure on top of said upper cladding layer by depositing a further buffer layer and repeating steps (b) and (c) with respect to a further guiding layer, significant perturbation in light propagation within said upper waveguide structure being thereby prevented.
According to yet another aspect of the present invention, there is provided an integrated optical device comprising at least one structure having at least one waveguiding element, the device comprising:
a basic structure containing a substrate material carrying a buffer material layer coated with a core material layer of a higher refraction index as compared to that of the buffer layer;
said at least one waveguiding element formed in a guiding layer on top of said basic structure, wherein said guiding layer is made of a material with a refractive index higher than the refractive index of said buffer layer and the core layer, and is chosen so as to minimize a height of said at least one waveguiding element and to provide effective guiding of light in the core layer;
a cladding layer on top of a structure with said at least one waveguiding element, wherein a height difference between a height of the cladding layer region above said at least one waveguiding element and a height of the cladding layer region outside the waveguiding element is substantially small, the top cladding layer thereby having a desired flatness.
The device may comprise additional waveguides and additional loop-resonators, forming together several frequency selective switches, thereby providing complex optical signal switching and routing.
Since optical waveguides can be implemented in complex manners, the universal quantity characterizing the behavior of the confined light is the effective refractive index of the waveguide. In conventional devices, the difference between the effective refractive index of the waveguide and the index of the surrounding medium is typically smaller than 1%. When using ring micro resonator structures, the effective refractive index of the ring waveguide has to be large, i.e., typically greater than 20%, to accommodate tight mode confinement and small losses. In these structures, however, the effective index of the ring waveguide and the linear waveguide are similar to within few percents (e.g., about 3%). The present invention provides for using several (at least two) ring resonators (ring waveguides) in an integrated optical device, the refractive index of the ring waveguide being thereby substantially greater (e.g., 20% greater) than the refractive index of the linear waveguide that receives an input signal.
In an optical complex filter/resonator according to the invention, waveguide sections are specifically connected to ring resonators in a configuration which enables realization of optical switching, wavelength routing, optical filtering, etc. The device may also continue a plurality of such filters in a wavelength router module.
Modern optical communications are typically based on transmitting frequency multiplexed optical signals through an optical fiber. The OADM is capable of adding or dropping optical channels from an optical fiber, and is an essential element in modern optical communications. In the present invention, the OADM is based on a combination of tunable filters, which provide the add or drop multiplexing functions. Since OADMs have to meet stringent criteria in their filtering, each ring resonator is an optical filter, and, by combining them in parallel, high order filters are obtained.
In general, the resonator-cavity loops (ring-resonators) can be replaced by any other implementation of a frequency-selective element that couple between the two waveguide sections. For example, optical gratings can be used.